Diabetes can have both a daily and long-term impact on people with the condition – both physically and in terms of its psychosocial effects. It is now known that people with diabetes are at a substantially increased risk of experiencing mental distress, particularly depression. Furthermore, a growing bank of evidence points to a two-way relationship between the conditions. Yet depression is often under-diagnosed in people with diabetes. Cathy Lloyd describes the effects of diabetes on depression and vice versa, and looks at the effectiveness of available tools for detecting depression in people with diabetes.